Red Crown Rot of Soybeans

INSIGHTS

Figure 1. County level confirmation provided by verbal communications from Dr. Steven Clough USDA-ARS, Diana Plewa University of Illinois Plant Clinic, Whitney Welker (Student), Dr. Carl Bradley University of Kentucky and Dr. Darcy Telenko Purdue University

Figure 2. Interveinal chlorosis and necrosis caused by red crown rot of soybeans. Image by A. Peterson, IL Soybean Assoc.

• Red crown rot is a soybean fungal disease recently observed in Midwestern states caused by early season infection of C. ilicicola.

• Key diagnostic feature is reddish discoloration of stem at soil line that may include bright, red-orange colored perithecia or reproductive structures appearing as tiny red balls.

• Management options include using a seed treatment, crop rotation, controlling root nematodes, and potentially delayed planting.

Red Crown Rot is a relatively newer soybean disease observed in the Midwest (Figure 1) that is caused by a soil-borne fungal pathogen, Calonectria ilicicola (anamorph: Cylindrocladium parasiticum). Red crown rot was first observed in Illinois in 2017 but occurs worldwide in warm- temperate and tropical regions. The same pathogen also impacts other crops such as peanuts, ginger, and blueberries and has been found in the Southern U.S. since the 1970s. Red crown rot is characterized by the fungal reproductive structures on the crown or lower stem at the soil line which give a reddish appearance.

Infection

C. ilicicola can survive as microsclerotia in the soil for several years. Infection is favored by wet conditions around planting and the disease depletes resources in the roots and stem of the soybean plant. Warm, wet growing season conditions will continue to drive progression of the disease. Symptoms will often first show up in poorly drained or low areas of the field. Soil temperatures of 77°-86° F favor disease development, but infection decreases as soil temperatures increase above 86°F. Foliar symptoms will appear later in the season as toxin from the pathogen begin to accumulate in the leaves, causing interveinal chlorosis and necrosis. Secondary disease spread can be caused by the ejection of mature ascospores from the perithecia (reddish, orange structures) on the stem from rainwater splash or runoff. C. ilicicola microsclerotia are spread by the movement of infested plant debris or soil particles carried by wind, equipment, or livestock.

Figure 3. Red discoloration at soil line on a soybean stem caused by red crown rot. Image by N. Prater, Syngenta

Figure 4. Red-orange colored perithecia caused by red crown rot of soybeans. Image by N. Prater,Syngenta

Symptoms

• Fungal symptoms usually don’t appear until later at the R3 to R7 growth stages.

• Infected plants or small patches are generally found randomly throughout the field.

• Key diagnostic feature is reddish discoloration of stem at soil line that may include bright, red-orange colored perithecia or reproductive structures appearing as tiny red balls (Figures 3 and 4).

• The vascular tissue inside the stem may appear gray brown in color (not white) and the roots of the plant may become rotted in later stages of disease development.

• Yellowing of leaves can occur after R3 growth stages but may not always be observed. Severely affected plants will prematurely senesce although leaves will remain attached afterwards.

• This disease has similar foliar symptoms to sudden death syndrome (SDS) and brown stem rot (BSR) where interveinal chlorosis then necrosis appears (yellow blotches between leaf veins – Figure 2). It is important to inspect stems and roots to help determine the causal pathogen.

• Late season disease identification of dead plants may be challenging. For help identifying red crown rot in soybeans send a plant sample to your state’s plant pathology identification lab.

Management

• Yield potential can be significantly impacted (up to 30% has been documented in fields where red crown rot has been present for years).

• Crop rotation to a non-host crop for one or more years may help decrease inoculum in the field and decrease risk of red crown rot presence.

• Delayed planting may help lessen the severity of red crown rot infection.

• Sanitize equipment before leaving a field if there is known infection to help avoid mechanical transmission.

• Manage root damaging nematodes to help lessen the access points for this fungal pathogen.

• A few seed treatments have recently been granted Section 2(ee) labels in specific states to help manage against early seedling infections. As of 11/3/23, Saltro^® seed treatment has received special labels for Arkansas, Illinois, Indiana, Iowa, Kentucky, Missouri, and Tennessee for suppression of Red Crown Rot. On farm trials in 2023 have shown delayed disease development and improved yields from Saltro when Red Crown Rot was present (Figure 5). Always refer to product label for the most current information.

Figure 5. Saltro performance on red crown rot in soybeans (1.428oz/unit) compared to a competitor seed treatment; Split planter field design; August 16, 2023, Southern Illinois; See the difference – Saltro seed treatment +8.8bu/A compared to competitor seed treatment.
On Farm Trials – Southern IL – 8/16/2023
MCG – 08/2023. Results may vary by geography.
Product performance assumes disease presence. Performance assessments are based upon results or analysis of public information, field observations and/or internal Syngenta evaluations. Trials reflect treatment rates commonly recommended in the marketplace. All photos are either the property of Syngenta or are used with permission.